The principal delay in cardiac depolarization impulse as it passes from the SA node to ventricular myocardial cells occurs where?

• A. SA Node
• B. AV Node
• C. Bundle of His
• D. Purkinje Fibers

Answer: (B)

The principal delay in the transmission of the cardiac depolarization impulse from atrial to ventricular tissue occurs at the AV node. The nodal tissue conducts impulses more slowly than the atrial myocardium or the His-Purkinje system because its cells have slower upstrokes (calcium-based action potentials) and fewer gap junctions, yielding a deliberate pause. This pause—about a fraction of a second—gives the atria time to contract and finish filling the ventricles before the impulse surges rapidly through the His bundle and Purkinje system to cause coordinated ventricular contraction. Autonomic input can modulate this delay: parasympathetic activity lengthens it, while sympathetic activity shortens it.